Trimmer condenser



.im 1, 1963 F. R. YOUNG 3,071,716 1 TRIMMER CONDENSER Filed April 19. 1960 Till-IM '41 m Qt,

' INVENTOR.

United States Patent O 3,071,716 TRIMMER CONDENSER Fletcher R. Young, North East, Pa., assignor to Erie Resistor Corporation, Erie, Pa., a corporation of Penn- Sylvania Filed Apr. 19, 1960, Ser. No. 23,285 9 Claims. (Cl. 317-249) This invention is intended` to improve the stability of tubular trimmer condensers by decreasing the air gap between the plunger and the bore of the dielectric and thereby minimizing changes in capacity. 'In a preferred form, the plunger is coated with an excess of solder and is assembled into the tubular dielectric while the solder is in a molten condition. During assembly, the excess solder squeezes out between the plunger and the dielectric at either end of the plunger and the remaining solder upon cooling shrinks slightly away from the dielectric and provides a very close sliding t. Air gaps between the plunger and dielectric of between ten and one hundred micro inches are readily obtainable.

In the drawing, FIG. 1 is a longitudinal section through a trimmer, FIG. 2 is a section on line 2 2 of FIG. l, FIG. 3 is a longitudinal section through another trimmer, and FIG. 4 is a section on line 4-4 of FIG. 3.

The trimmer condenser has a tubular dielectric 1 and a metallic plunger 2 longitudinally slidable in the bore of the dielectric. The adjusted capacity of the trimmer is determined by the amount of overlap of the plunger 2 and 'a metal electrode 3 at one end of the dielectric. The dielectric and plunger are selected from materials in which Ithe coeiiicients of thermal expansion are matched so that changes in dimensions with temperature have a minimum of effect upon the capacity. Vycor glass is a suitable dielectric when the plunger is Invarf Other combinations rare available. The dielectric may be chosen to have 3,071,716 Patented Jan. 1, 1963 ice n by manufacturing or other variations in the size of the a dielectricv constant which varies with temperature so that the resultant capacity at any setting will vary with temperature.

The trimmer is mounted by a metal bushing 4 havin one end 5 telescoped within the dielectric'and having a ange 6 butting against the end of the dielectric and secured thereto by a solder bead 7 connecting theflange and an external metal coating 8 on the dielectric. The opposite end 9 `of the bushing is externally threaded to receive a nut 10 by which the chassis 11 is clamped between the nut and the ange to securely mount the trimmer; A lock washer 12 prevents loosening of the nut. At the opposite end of the dielectric is an end cap or closure 13 and a lead 14, both soldered to the electrode 3. Suitable insulation may be provided to prevent metal to metal contact between the plunger and the end cap. In

many cases the end closure is omitted.

The longitudinal position of the plunger '2v is adjusted by a metal spindle 15 journaled in the bushing 4 and having a snap ring 16 engaging the outer end Y17 of the bushing and a collar 18 spaced from the inner end 19 of the bushing. A spring washer 20 arranged between the inner end of the bushing and the collar exerts an axial force holding the snap ring 16 tight against the outer end 17 of the bushing preventing axial movement of the spindle and plunger. The spindle is provided with a screw driver slot 21 for turning. The inner end of the spindle is slotted at 22 and is externally threaded at 23 to engage internal threads 24 in the plunger 2. yunstressed position, the outside diameter of the threaded section 23 is greater than the inside diameter of the threads 24 in the plunger so that the threaded section 23 is slightly compressed as it is threaded into the bore of the plunger and this compression is suiiicient to maintain a tight engagement of the threads under all conditions. This means that the axial position of the plunger can be In the threads.

As shown in FIG. 2, the bore of the dielectric is non circular, there being one or more flats or ribs 25. Also, as shown in FIG. 2, the plunger is non circular and complementary to the bore of the dielectric so that the plunger does not rotate as it traverses the bore of the dielectric.

The space between the plunger 2 and the bore of the dielectric 1 is filled by a solder coating 26. In manufacture, the exterior of the plunger 2 is coated with more than enough solder to lill the gap between the plunger and the bore of the dielectric and the dielectric and plunger are both heated above the melting point of the solder. Upon insertion of the plunger into the bore of the dielectric, the excess of molten solder over and above that required to ll the gap is squeezed out either end of the plunger and only the amount of solder required to lill the gap remains. Since solder has a greater coefficient of expansion than either the plunger or dielectric, upon cooling, the solder solidiiies and shrinks slightly away from the bore of the dielectric, providing suicient air gap to permit traverse of the plunger. The plunger may be circular or non circular before the solder coating is added. The solder coating conforms to the glass. Starting with a plunger having an outside diameter several thousandths of an inch less than the inside diameter of the bore of the dielectric, the solder coating in the molten condition will completely ll the gap and upon cooling, will leave an -air gap of from ten to one hundred micro inches. This is substantially less than the air gap which can be obtained practically by machining methods. Because of this exceedingly small air gap, there is substantially no chance of varying the capacity of the condenser by lateral shifting of the plunger relative to the bore of the dielectric. When the air gap is large, lateral shifting of the plunger is possible and the capacity will vary, depending upon whether the plunger is perfectly concentric or shifted to one side of the center of the bore of the dielectric. The solder coating is particularly adaptable to dielectrics having non circular bores due to the presence of internal flats or ribs 25 or to other non circular constructions. In all of these, the molten solder conforms to the gap between the plunger and the non circular bore and provides a uniformly small resultant air gap.

The manner of heating the dielectric and the plunger prior to assembly is not critical. It is possible to heat the dielectric and plunger on a hot plate. It is also possible to perform the heating by dipping in hot oil, preferably one of the tinning oils because of the smoothing effect it has on the solder.

For the Invar-Vycor and similar combinations, soft lead, tin, solder alloys having a melting point of 360 F. and a hot oil bath having a temperature of 450 F. have been used. Other materials may be used. With dielectrics which will stand higher temperature, e.g. quartz, a fusible metal with a higher melting point, such as silver solder, may be used. The coeificient of thermal expansion of the fusible metal should be greater than the co- 'efficient of the dielectric so that upon cooling the fusible metal will shrink and leave a slight air gap.

In summary, the requirements are (l) matching coeicients of thermal expansion of the dielectric and the plunger, (2) a fusible metal coating on the plunger having a coeiiicient of thermal expansion higher than and a melting point lower than the dielectric or plunger, and (3) the fusible metal coating must wet or adhere to the plunger but not the dielectric. The dielectric is illustrated as the outer member and the plunger as the inner member. The relative positive of these members may be reversed in which case the fusible metal coating will be on the inner surface of the plunger but the mode of operation will be unchanged.

The solder coating resulting from the assembly is not necessarily perfectly polished or smooth. Upon eX- amination, it may have a rippled appearance but this is immaterial. The high spots on the solder coating which provide the ripple finish ride on the bore of the dielectric and maintain the plunger centralized. The fact that these high spots have a clearance of from ten to one hundred micro inches is the important factor in centralizing the plunger. The existence of other spots in the solder coating having a greater clearance does not effect the stability of the capacitance.

In the trimmer of FIGS. 3 and 4, the dielectric tube 27 has a circular bore and the cup shaped plunger 28 has circular side walls 29 which are manufactured from one to several thousandths less in outside diameter than the inside diameter of the bore of the dielectric. These may be made of the same materials as the FIGS. 1 and 2 trimmer, eg. Vycor for the dielectric and Invar for the plunger. Prior to assembly, an exterior solder coating 30 is applied to the plunger and the dielectric and plunger are heated to a temperature sulllcient to melt the solder and then assembled. The excess solder squeezes out either end and provides the extremely small air gap desirable for stability of capacity under all conditions. The external surface of the solder coating on the plunger 28 conforms to the bore of the dielectric and is accordingly circular. This means that the plunger can rotate relative to the dielectric as it traverses the bore.

In use, the chassis or supporting frame 31 is clamped between a ange 32 on a body 33 and a nut 34 threaded on one end 35 of the body. The dielectric is secured to the body by a solder bead 36 between the flange 32 and a metallic coating 37 on the dielectric. The inner end 38 of the body extends within the bore of the dielectric and is provided with a recess 39 receiving a coil spring v 40 arranged between the bottom 41 of the recess and a nut 42, slidably received in the recess and having tangs 43 received in slots 44 and holding the nut against rotation. An adjusting screw 45 is threaded in the section 46 of the body and also in the nut 42 so that the tension of the spring 40 takes up any slack between the threads of the adjusting screw and the section 46. This prevents shifting of the adjusting screw relative to the body. The outer end of the adjusting screw is provided with a head 47 and'screw driver slot 48 and is received within a recess 49 in the outer end of the body 33. The opposite end of the adjusting screw is fixed to the bottom wall 50 of the plunger. As the adjusting screw is turned, the plunger is also turned and is moved axially within the bore of the dielectric. The position of the plunger relative to an external electrode 51 on the dielectric determines the adjusted capacity of the trimmer which appears between the chassis 31 and a lead 52 soldered to the electrode 51. The position of minimum capacity is determined by engagement of the rim 53 of the plunger With a shoulder 54 on the body 33. The positionof maximum capacity of the trimmer is determined by engagement of the head 47 of the adjusting screw with the bottom 55 of thevrecess 49.

Both trimmers are stable under vibration, changes in temperature and other conditions encountered in service.

What is claimed as new is:

l. A trimmer condenser having inner and outer electrodes, a tubular dielectric carrying one of the electrodes, the other electrode being a plunger in telescoping relation to said dielectric with a gap between one surface of the plunger and the dielectric, traversing means for the plunger, and a fusible metal coating on saidrsurface ofv the plunger of suicient thickness in the molten state to lill the gap between the plunger and dielectric and said fusible metal adhering to the plunger but not to the dielectric, the plunger with its fusible metal coating having a coeliicient to thermal expansion greater than the dielectric whereby the shrinkage upon cooling provides an air gap between the coating and the bore of the dielectric and the melting point of said fusible metal being less than the melting points of the plunger and dielectric.

2. The trimmer of claim 1 in which the coelcient of thermal expansion of theplunger and dielectric are substantially equal and are less than the coelicient of thermal expansion of the fusible metal. p

3. The trimmer of claim l in which the bore of the dielectric is non circular.

4. The trimmer of claim l in which the dielectric is quartz and the fusible metal is silver solder.

5. The method of improving the temperature stability of trimmer condensers of the type having inner and outer electrodes, one of the electrodes being carried by a tubular dielectric and the other electrode being a plunger in telescoping relation to the dielectric with a gap between one surface of the plunger and the dielectric, which com- -prises coating said surface of the plunger with fusible metal of coeicient of thermal expansion greater-than the plunger ,and dielectric, the fusible metal coating being in excess of that required to fill the gap between the plunger and dielectric in the molten state of the fusible metal and said fusible metal adhering to the plunger but not to the dielectric and the melting point of said fusible metal being less than the melting points of the plunger and dielectric, heating the dielectric and the coated plunger above the melting point of the fusible metal, and telescoping the plunger and dielectric so the molten fusible metal conforms to the gap and the excess fusible metal squeezes out of the gap and the fusible metal remaining in the gap upon cooling shrinks away from the bore of the dielectric to provide an air gap.

6. The method of claim 5 in which the bore of the dielectric is non circular and the fusible metal provides a conforming non circular surface on the plunger.

7. The method of claim 5 in which the fusible metal is soft solder and the dielectric and the coated plunger are heated by dipping in hot tinning oil.

8. .A trimmer condenser having inner and outer electrodes, a tubular dielectric carrying the outer electrode, the inner electrode being a plunger in the .bore of the dielectric of lesser diameter than said bore, traversing l means for the plunger, and a soft solder coating on the plunger of `sufficient thickness inthe molten state to ll the gap between the plunger and dielectric and the solder coating adhering to the plunger but not to the dielectric, the plunger with its solder coating having a coellicient of `thermal expansion greater than the dielectric Y whereby the shrinkage upon cooling provides a gap between the coating and the bore of the dielectric.

9. The trimmer of claim 8 in which the bore of the dielectric is non circular. f

References Cited in the tile of this patent UNITED STATES PATENTS 2,748,327 Wadsworth May 29, 1956 2,910,635 Abrams Oct. 27, 1959 FOREIGN PATENTS 560,372 Great Britain Mar. 3l,y 1944 :159,463 Australia cv Oct. 25, 1954 

1. A TRIMMER CONDENSER HAVING INNER AND OUTER ELECTRODES, A TUBULAR DIELECTRIC CARRYING ONE OF THE ELECTRODES, THE OTHER ELECTRODE BEING A PLUNGER IN TELESCOPING RELATION TO SAID DIELECTRIC WITH A GAP BETWEEN ONE SURFACE OF THE PLUNGER AND THE DIELECTRIC, TRAVERSING MEANS FOR THE PLUNGER, AND A FUSIBLE METAL COATING ON SAID SURFACE OF THE PLUNGER OF SUFFICIENT THICKNESS IN THE MOLTEN STATE TO FILL THE GAP BETWEEN THE PLUNGER AND DIELECTRIC AND SAID FUSIBLE METAL ADHERING TO THE PLUNGER BUT NOT TO THE DIELECTRIC, THE PLUNGER WITH ITS FUSIBLE METAL COATING HAVING A COEFFICIENT TO THERMAL EXPANSION GREATER THAN THE DIELECTRIC WHEREBY THE SHRINKAGE UPON COOLING PROVIDES AN AIR GAP BETWEEN THE COATING AND THE BORE OF THE DIELECTRIC AND THE MELTING POINT OF SAID FUSIBLE METAL BEING LESS THAN THE MELTING POINTS OF THE PLUNGER AND DIELECTRIC. 